This invention relates to a multiple speed dynamoelectric machine, and more particularly to a multiple speed permanent split capacitor (PSC) or other capacitor-run multi-speed induction motor, such as may be utilized to directly drive a fan mounted on the rotor shaft of the motor.
Generally, in a motor application for use with a direct drive overhead ceiling fan or the like, the motor is designed to run at a relatively slow speed. For example, a multiple-speed overhead ceiling fan motor may be operated at a maximum speed of about 350 rpm and at a minimum speed of about 50 rpm. These motors generally are multiple pole motors having either 12 or 18 poles, as compared to more conventional 2, 4, 6, or 8 pole motors. A PSC motor is a capacitor run motor that uses its auxiliary winding and capacitor continuously, without change in the capacitance. No starting switch or relay is required to switch out the auxiliary winding of the motor once the motor has attained its operational speed. Typically, a PSC motor comprises a main winding adapted to be connected across two AC power lines. An auxiliary winding and a permanent running capacitor are wired in parallel to the main winding so that upon energization of the windings, the main winding and the auxiliary winding are spaced 90 electrical degrees apart. A squirrel-cage-type rotor is usually utilized in PSC induction motors.
In a number of PSC fan motor applications, it is highly desirable to provide for multiple speed operation of the fan motor. Speed control of a PSC motor is typically obtained by adjusting the flux of the motor and thereby changing the slip. As a general rule, pole changing is not utilized with PSC motor applications. There are four basic methods of changing the flux of the stator and thereby changing the slip and operating speed of a PSC motor. A first method of speed control of a PSC motor utilizes a change of connections of the main windings. Secondly, the voltage impressed on one or both the main and the auxiliary winding may be changed. Thirdly, an external impedance or capacitance may be placed in series with the main winding of the motor. And, fourthly, various tapping methods using autotransformers and secondary main windings are used.
Looking first at voltage change methods for speed control, these methods typically employ a simple autotransformer used in conjunction with the PSC motor such that the voltage on both the main winding and the auxiliary circuit (i.e., the permanent capacitor and the auxiliary winding) is the same at all times, but the actual voltage applied to the main and auxiliary windings is varied depending on the tapping point of the autotransformer.
In utilizing voltage changes for speed control purposes, the PSC motor is normally operated at its high speed mode of operation when the full voltage of the AC power supply is utilized to energize both the main and auxiliary windings of the motor. For example, in an overhead ceiling fan motor application using voltage speed control, the full AC line voltage (120 volt) would be impressed across the windings resulting in high speed operation of the motor (e.g., 350 rpm). For medium speed operation of the motor, the autotransformer in parallel with the main and auxiliary windings is tapped at an intermediate point so as to reduce the voltage impressed on both the main and auxiliary windings with a corresponding speed-torque curve reduction such that the torque of the motor is less than at its high speed operation with the drag or slip of the fan blades resulting in a steady state operating speed of the motor slower than its high speed mode of operation. Likewise, slower speed modes of operation may be obtained by further reducing the voltage (and hence the torque) of the motor. In voltage speed control systems, the operating speed of the motor generally depends on the load applied to the motor. Also, the locked rotor torque of the motor is necessarily low when the motor is operated at its low speed mode of operation and the low speed connection is inherently unstable because the fan-torque and motor-torque curves intersect one another at a very small angle such that the motor is sensitive to changes in voltage and in load.
If an external impedance is utilized for speed control purposes, the impedance typically is either a resistor or a reactor which is connected in series with the main winding such that the voltage impressed across the main winding is reduced when the impedance is connected in series to the main windings thereby reducing the flux and increasing the slip of the motor and in turn reducing the operating speed of the motor. In the coassigned U.S. Pat. No. 4,408,150, a capacitor is interconnected in series with the main winding of the motor and which, when serially connected to the main winding of the motor, results in a reduction in speed of the motor.
In tapped winding speed control arrangements, such motors typically effect speed control by flux control, accomplished primarily by changing the impressed volts per turn on the main winding. Generally, tapped winding motors vary the volts per turn (and hence the flux, slip, and speed of the motor) by changing the number of series conductors in the main winding. For example, a two speed tapped winding motor utilizes three windings including a main winding, an extra main or intermediate main winding, and an auxiliary winding. The main and intermediate main windings are wound in space phase with one another (i.e., one is wound on top of the other) in the same slots, with the same distribution but not necessarily with the same number of turns or wire size. For more than two speeds, the intermediate main winding itself is tapped.
In a recently commercially available PSC direct drive fan motor, only one main winding was provided and the auxiliary winding was tapped at different locations. Through the use of a double pole, triple throw speed selector switch, selected physical poles of the auxiliary winding could be electrically removed from the remainder of the auxiliary winding in parallel with the main winding, or the tapped auxiliary winding could be utilized as a voltge divider thereby to change the flux impressed upon the windings of the motor. However, because one or more of the physical poles of the auxiliary winding of this motor is not energized at the intermediate or slower speeds of operation of the motor, the flux distribution of this motor is not balanced around the stator core and this unbalanced magnetic flux results in noisy operation of the motor at slower speeds.